In radio communication networks it is known to utilize multi-antenna techniques to increase performance. For example, in the case of the LTE (Long Term Evolution) cellular radio technology specified by 3GPP (3rd Generation Partnership Project), multi-antenna transmission techniques may be applied by operating multiple antennas of a base station, in the LTE radio technology referred to as “evolved Node B” (eNB), in a transmit diversity mode or in a spatial multiplexing mode (see, e.g., 3GPP TS 36.211 V12.4.0).
When using such multi-antenna techniques, the transmitted signal from each antenna may be controlled in such a way that the received signal from each antenna adds up coherently at a certain receiver, e.g., a user equipment (UE). In this way, the overall transmit signal from all antennas results in a received signal power which is relatively increased in one or more spatial directions, i.e., forms one or more beams. Such control is referred to as beamforming.
For efficient utilization of beamforming, accurate knowledge of signal quality associated with each beam is an important aspect. At the eNB such knowledge may for example be obtained by letting the UE transmit a pre-determined reference signal, measuring this reference signal as received at the multiple antennas, and estimating the signal quality associated with a given beamforming configuration on the basis of the measured reference signals, assuming reciprocity of uplink and downlink transmission direction. Further, a predefined codebook may be applied, which defines which beamforming configuration to apply depending on feedback from the UE. Such feedback may for example include reports of channel quality as measured by the UE while performing transmissions on a certain beam.
However, in certain scenarios, the known ways of controlling the beamforming configuration as applied with respect to a certain UE may provide unsatisfactory results, due to estimates of the signal qualities which the selection of the currently applied beamforming configuration is based on being outdated or not sufficiently accurate. This may for example result in selecting a beamforming configuration in which the beam “misses” the UE, which means that the performance experienced by this UE may be even less than without utilization of beamforming. This problem becomes worse as the beams become more focused by utilization of a larger number of antennas. On the other hand, there are also limitations with respect to improving the accuracy of the estimates of the signal qualities. For example, once a certain beam is defined for transmissions to the UE, the UE will only perform measurements on this beam, and accurate assessment of the signal qualities which could be achieved by other beams may not be possible. Further, also the temporal density and power at which the UE may transmit reference signals to the eNB is typically limited, and estimating signal quality which can be achieved in the downlink direction on the basis of measurements on signals transmitted in the uplink direction may introduce various kinds of systematic errors.
Accordingly, there is a need for techniques which allow for efficiently utilizing beamforming for radio transmissions.